Because of the urgent need to develop novel effective therapies for pancreatic adenocarcinoma (PDAC), this proposal will test the novel hypothesis that a 212Pb-labeled mAb-based radioimmunotherapy (RIT) targeting a B7-H3 epitope selectively expressed on PDAC cells will enhance the ability of gemcitabine and the Sonic hedgehog (SHH) inhibitor LDE225 to eradicate PDAC, by eliminating not only differentiated PDAC cells, but also pancreatic cancer initiating cells (CICs). The combination strategy we plan to optimize takes advantage of the unique specificity of mAb 376.96. This mAb recognizes an extracellular epitope of the coinhibitory molecule B7-H3 which is expressed on human PDAC cell lines and in surgically removed PDAC lesions, including pancreatic CICs, but has a restricted distribution in normal tissues. This proposal will test the hypothesis that PDAC can be eradicated by targeting not only differentiated/differentiating pancreatic cancer cells, which form the bulk of the tumor, but also pancreatic CICs, the root of the disease, with 212Pb conjugated to the novel mAb 376.96. This hypothesis stems from the cancer stem cell theory which postulates that CICs have to be eliminated to cure a malignant disease, since CICs are responsible for disease recurrence and metastatic spread. The validity of our combination strategy is supported by our preliminary results: as shown in Figs. 4, 5, and 6 mAb 376.96 enhances the anti-proliferative activity of gemcitabine and its ability to eliminate CICs. Gemcitabine is currently the standard chemotherapeutic agent for PDAC. However, the effect of gemcitabine alone is limited and most patients develop resistance to the therapy. This resistance may reflect the chemo-resistance of CICs. Targeted RIT with 212Pb, which decays to 212Bi, will be combined with gemcitabine and LDE225, an inhibitor of the SHH pathway, since this inhibitor has been shown to be effective in reducing pancreatic CICs. The specific aims of this proposal will test whether: i. B7-H3-specific mAb 376.96 labeled with 212Pb binds to human PDAC cell lines and inhibits in vitro proliferation of human pancreatic CIC tumorspheres alone or in combination with gemcitabine and LDE225. ii. B7-H3-specific mAb 376.96 labeled with 212Pb, in combination with gemcitabine and LDE225 eradicates orthotopic disease in immunodeficient mice grafted with human PDAC cells, since this combination eliminates not only differentiated PDAC cells, but also pancreatic CICs. iii. B7-H3-specific mAb 376.96 labeled with 212Pb, in combination with gemcitabine and LDE225, eradicates micrometastatic disease in immunodeficient mice grafted with human PDAC cells. The information derived from the outlined studies will provide a solid foundation for clinical translation of the proposed 212Pb-376.96 antibody and CIC therapy to treat pancreatic cancer.